Pipeline booster stations are used in long gas pipelines to maintain a desired discharge pressure, suction pressure, and/or flow rate of the gas in a pipeline. Generally a pipeline booster station receives a cool decompressed gas stream and employs centrifugal compressors to produce high volume flow with relatively low ratio of discharge to suction pressure. Since these booster stations are typically unattended offshore platforms or located in remote land areas, automatic shutdown systems are typically employed which operate to shutdown the station if an abnormal operating condition is detected.
One can manipulate the outlet flow of a centrifugal compressor by suction throttling, discharge throttling, variable inlet guide veins or variable speed such that a desired outlet flow can be maintained. However, the centrifugal compressor control system is not complete without consideration of surge control. A typical example of surge control involves cooling a portion of the compressed gas in a recycle cooler for recycle to the compressor's suction inlet. In this type of surge control a desired minimum temperature for the recycle gas and a desired minimum suction flow rate for the compressor are maintained to insure operation of the compressor that is safe from surge.
In operation of a typical pipeline pressure booster station, cold decompressed gas, e.g. 800 psi and 27.degree. F., flowing into the booster station is raised in pressure to a desired pressure of about 2,000 psi by one or more compressors. Unfortunately, however, the act of compressing the gas to raise the pressure to a desired value also raises the temperature of the gas to an undesirable value, such that discharging the compressed gas from the compressor outlet directly into the pipeline at an elevated temperature could aid other factors, such as corrosion, in causing cracking in the pipeline. Therefore, the compressed gas at the compressor outlet should not be reinjected into the pipeline until its temperature is reduced if the possibility of avoiding cracks in the pipeline is to be maximized.
As in any commercial process, it is desirable that the cooling process be carried out while minimizing the equipment required for carrying out the cooling process. This is especially true in pipeline pressure booster stations since the stations are typically located offshore or in remote land areas where sheltered areas and commercial electric power are at a premium.
It is thus noted that the recycle stream and the recycle cooler utilized for surge control of the compressor are typically sized to handle about 70 percent of the compressor capacity. However, in normal operation of the pipeline, the compressor generally carries more than 70% of its rated capacity and therefore the recycle cooling capacity is not needed for surge control. The maximum cooling capacity of the recycle cooler could only be required during startup or testing of the compressor, and during these operations temperature reduction requirements for the gas discharged from the booster station into the pipeline are minimal due to the low suction inlet flow, and correspondingly low compressor discharge flow.
It is thus an object of this invention to provide apparatus and method for reducing the temperature of a relatively warm compressed gas stream before the gas is discharged into a relatively cool pipeline by using excess cooling capacity of the compressor's recycle cooler to cool the gas before discharging the compressed gas into the pipeline.
It is another object of this invention to cool a portion of the main compressed gas stream and to recombine the cool gas stream with the main compressed gas stream so as to reduce the temperature of the compressed gas discharged from the booster station such that gas is reinjected into the pipeline at a desirable temperature.
It is a still further object of this invention to control the temperature of the gas discharge into the pipeline without interfering with the compressors surge control scheme.